With the advance of photolithography techniques, the features of patterns written on the photoresist layer of a substrate become increasingly smaller.
The ability to project a clear and fine pattern of small details onto a wafer in the photolithographic process is limited by the wavelength of the irradiation used, and the ability of the reduction optics to capture enough diffraction orders from the illuminated mask (affected by the numerical aperture of the optical lens used).
Variations in the critical dimensions (CD)—that is, variations in the feature sizes on the manufactured wafer, with respect to their expected values may impair the process window and the yield.
Various techniques were introduced to overcome variations in CD. For example, optimizing the exposure dose of the irradiation directed on the photomask, to better match between the expected CD and the actual CD of different parts of the resulting pattern on the wafer.
Another way of dealing with CD variations involves planting shading elements in specific positions within the photomask substrate, above the pattern parts with variable optical density.